Toll-like receptors (TLRs) are highly conserved proteins which play an integral role in innate and adaptive immunity. The nine characterized mammalian TLRs are pattern recognition receptors for integral pathogen associated molecular patterns (PAMPs), providing a means to discriminate between self and non-self. Ligation of TLRs activates cells of the innate immune system, inducing functions such as phagocytosis and IFN-a and IFN-b secretion. TLR-ligation also serves as a bridge between the innate and adaptive immune systems. TLR stimulation of DCs and macrophages induces the production of inflammatory cytokines such as IL-12, and upregulation of MHC class II and costimulatory ligands such as CD40 and CD86. While roles if or TLRs on APCs are well-established, surprisingly little is known about their expression and potential function on T cells. New studies from our laboratory show that several TLRs are expressed following activation of na?ve CD4+CD25- T cells. Ligation of these TLRs by their appropriate PAMPs activates several known downstream signaling pathways, including NF-kB, p38 JNK, and ERK. Functionally, this leads to enhanced T cell survival and may also provide costimulation with sub-optimal TCR ligation. Our workinq hypothesis is that TLRs signaling on T cells is an important contributor to cellular immune responses, and a block to the development of tolerance. To this end, the long-term goals of this research program are to define the expression and function of TLRs on T cells. In specific aim #1 we will characterize the signals which govern the expression and upregulation of TLRs on T cells, and the downstream signaling pathways which TLRs use to mediate their effects. Specific aim #2 will use cells from TCR transgenic mice coupled with in vitro and in vivo adoptive transfer systems to conduct a detailed examination of the effects of TLR activation of T cells on induction of activation and costimulatory molecules, cytokine production or skewing, and effector/memory differentiation and responsiveness. This approach will allow for precise definition of what TLR stimulation of T cells can do. In specific aim #3 we will then ask which if the functions identified for TLRs in aim #2 are operative during in vivo immune responses. To do so, we will create in vivo models in which only T cells are deficient in TLR signaling, in order to assess the specific role of TLR signals on T cells during in vivo immune responses. Specific models include adoptive transfers, bone-marrow chimeras, and transplantation using TCR transgenic mice on TLR3- or MyD88-deficient backgrounds [unreadable] [unreadable]